1. Field of the Invention
The present invention relates to an amplification-type solid-state image sensor, particularly to a threshold voltage modulation-type solid-state image sensor using a junction gate type field-effect transistor whose threshold voltage is modulated in accordance with an incident light as a pixel.
2. Description of the Related Art
In recent years, a solid-state image sensor referred to as an amplification-type has intensively been developed in which a charge detection circuit is disposed for each pixel. Above all, a solid-state image sensor referred to as a threshold voltage modulation-type has been noted. In the solid-state image sensor, a plurality of pixels including junction-type field-effect transistors (hereinafter referred to as FET) is arranged in a matrix. Moreover, when light is incident, a signal charge is generated in each pixel, and the signal charges are accumulated in each pixel for a constant time. In each pixel, a change in a potential or threshold voltage is generated in a channel region of FET in each pixel in accordance with an accumulated amount of signal charges. Furthermore, a plurality of pixels is successively scanned following a predetermined order, and image signals are successively read, for example, by source follower type potential detection means.
A structure disclosed, for example, in FIG. 14 of Jpn. Pat. Appln. KOKAI Publication No. 8-78653 has heretofore been known as the threshold voltage modulation-type solid image sensor.
FIG. 1 shows a sectional structure of a device for one pixel of the sensor. An n-type layer 72 is formed in a p-type substrate 71, and an n+-type drain region 74 and source region 75 are formed in the surface region of the n-type layer 72 via a P+-type junction gate 73. Moreover, a MOS type gate electrode 76 for charge reset is formed adjacent to the junction gate 73.
In this solid-state image sensor, when light is incident, a plurality of electron-hole pairs is generated by photoelectric conversion. The electrons of the generated electron-hole pairs flow out to the drain region 74, and the holes are accumulated in the junction gate 73 to form the signal charge. Since the junction gate 73 is in a floating state, the potential of the junction gate 73 changes in accordance with the accumulated signal charge. Accordingly, the potential of the n-type layer 72 changes, and this is read out as the potential or current change of a source.
The solid-state image sensor shown in FIG. 1 is simple in the structure, and has a latent advantage suitable for miniaturization as compared with a charge coupled device (CCD) type. However, it cannot necessarily be said that the sensor is used in a broad range. Reasons are as follows.
That is, the junction gate 73 in which the signal charge is accumulated includes a P+-type region which contains p-type impurities with high density, and the junction gate 73 is potentially brought into the floating state. Therefore, when the MOS type gate electrode 76 is turned on, and the signal charge accumulated in the junction gate 73 is discharged to the p-type substrate 71, a remaining background charge exists without being discharged via the junction gate 73. This background charge amount changes by a so-called kTC noise in which a heat noise of on resistance of the MOS type gate electrode 76 is reflected. Therefore, even after the signal charge is reset, the background charge remains in the junction gate 73. A noise charge is superimposed upon the background charge, and this is read out as noise which appears on the signal.
Therefore, it is considered that the density of p-type impurities in the junction gate 73 is lowered in order to reduce the influence of the background charge. In this case, however, since the operation becomes unstable by a charged state of the chip surface, a capacitance between the junction gate 73 and n-type layer 72 drops, and a sufficient amount of signal charges cannot be accumulated, a problem of decrease of a saturated charge amount occurs. Therefore, there has heretofore been a demand for solving a problem that the saturated charge amount drops.